The light-addressable potentiometric sensor (LAPS) is a device developed to potentiometrically measure changes in pH, redox potential and transmembrane potential in a highly sensitive manner (see Hafeman et al., Science, Vol. 240, 1182 (1988)). The LAPS consists of an insulated semiconductor device which is immersed in an electrolyte. The sample of interest is placed on the surface of the insulator (at the insulator-electrolyte interface) and a bias voltage is generated such that the solution is negative with respect to the semiconductor. The insulator consists of a pH-sensitive material, such as silicon nitride or silicon oxinitride, or other charge-sensitive material. Electron-hole pairs are created in the semiconductor by a pulsed intensity-modulated light source, resulting in separation of charges in an area called the depletion region of the semiconductor. The electrons in the depletion region migrate, thereby causing a current of a magnitude that depends on the bias voltage and the chemistry of the sample at the adjacent insulator-solution interface. This movement of electrons yields a detectable alternating current in the external circuit.
An alternative measurement approach is based on the fact that the width of the depletion layer is related to the surface potential. The width of the depletion layer can be determined by measuring the capacitance between the semiconductor and the electrolyte.
A useful feature of using light excitation, is that different regions of the device may be monitored by selectively irradiating the location of interest, thereby allowing multiple potentiometric measurements with the use of multiple LEDs. The resultant photocurrent is monitored, giving a real-time measurement of, for instance, pH changes. Such measurements typically yield an RMS noise of 0.0005 to 0.001 pH units in a 1 Hz bandwidth. The primary utility of LAPS has been in commercial microphysiometers to measure the rate of proton excretion from cells by measuring the pH of a nearby surface (see McConnell et al., Science, Vol. 257, 1906 (1992)). The existing art in the field of LAPS provides for monitoring cell metabolism in terms of overall pH changes, redox potentials, and quantitative determinations of analytes for bulk samples. These systems, however, are not suitable for uses such as mapping charge distribution over very small areas or detecting chemical dynamics on the molecular level with high spatial resolution.
In the field of surface inspection several techniques have been developed for detecting charge on a surface. These methods exploit features of the general technique of scanning probe microscopy (SPM). One variant of SPM is based on an atomic force microscope (AFM) designed to map surface charge (for details about AFM see, e.g., Binning et al., Phys. Rev. Lett., Vol. 56, 930 (1986). Briefly, in traditional AFM a tip is mounted on a cantilever with a small spring constant and scanned over a surface such that the repulsive interatomic forces between the surface and the tip cause deflections in the cantilever. A feedback system is used to monitor and control the force between the tip and sample, and optical detection techniques such as interferometry or laser beam deflection are used to measure the resultant cantilever deflection during scanning.
An adaptation of AFM for SPM to map a charge distribution on a sample surface is discussed by B. D. Terris et al. in "Contact Electrification Using Force Microscopy", Physical Review Letters, Vol. 63, No. 24, pp. 2669-72 (1989). The charge measurement is based on the fact that the presence of charge on the scanned surface alters the spring constant of the cantilever. In the absence of any surface charge the typical spring constant is on the order of 0.2 N/m and the resonant frequency around 25 kHz. When the tip of the cantilever approaches a charge the tip-to-surface force gradient changes due to the Coulomb field. This change affects the cantilever's spring constant and consequently its resonant frequency. For efficient charge measurement an AC bias is applied between the tip and a back electrode to produce an oscillating charge on the tip. This enables one to measure surface charge via variations in the cantilevers oscillation response. For additional information refer to J. E. Stern et al., "Deposition and Imaging of Localized Charge on Insulator Surfaces Using a Force Microscope", Applied Physics Letters, Vol. 53, pp. 2717 (1988).
Improvements to SPM instrumentation have allowed for increased sensitivity and speed; however, there is no existing SPM that successfully scans a surface and measures its electrical properties in such a way that the measured signal is unaffected by incongruent tip-sample force interactions from topographic features. Additionally, the existing techniques do not allow one to measure charge with high sensitivity by scanning a sample while immersed in a fluid or gel.